Not once in my life have I heard or seen unions imterfering in any way or form with negotiations with an employer for better than union conditions. Unions negotiate global basics and of course allow better individual contracts.
Is it actually common for high-performing union members to have better deals? In my (limited) experience, the unions tend to advocate for the long-term membership, who are often below-average performers.
Yes. Usually wage ranges are negotiated. Negotiate before you join the company and make sure you understand your contract. If you are not sure, ask your union's representative or, of course, a lawyer specialized in labour law. Labour law is complex and you definitely want a specialist to answer your questions.
Don't trust the first google hits, especially if you are in the US. Disinformation by union busters is very common around here.
